The use of molds such as Penicillium roqueforti and Penicillium camemberti has a long history in the production of cheese, and their use as starter cultures goes back to the beginning of the 19th century. P. roqueforti is used as a secondary starter culture in the production of blue-veined cheeses, but it may also occur spontaneously in a number of other foods, and for some types of cheeses it is regarded as a contaminant. The fact that P. roqueforti is able to grow at high NaCl and low O2 concentration and at a relatively high CO2 concentration makes it suitable for the production of blue-veined cheese. During cheese maturation, P. roqueforti produces a number of extracellular peptidases and proteinases that are mainly responsible for the extensive proteolysis of blue-veined cheese. Large differences in proteolytic (136) and lipolytic activity (137) have been reported between different commercial strains of P. roqueforti. The lipolytic enzymes especially seem to be responsible for the characteristic flavor and taste of blue-veined cheeses through their production of high concentrations of methyl ketones (137). P. camemberti is used as a starter culture for the production of Camembert and similar types of surface-ripened cheeses, and it is highly restricted to the production of cheese and is seldom observed in other foods or in the environment (8). P. camemberti is able to grow on the cheese surface due to its high NaCl tolerance. On the cheese surface, it degrades lactate, resulting in an increased pH. When the lactate is depleted, the proteinases from P. camemberti then degrade casein, resulting in a further increase in pH and the release of ammonia. Besides its pronounced proteolytic activity, P. camemberti also produces lipases that are involved in aroma formation. Fig. 3 shows the mycelial growth of P. camemberti on a soft cheese surface.
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